1. Field of the Invention
The present invention relates to a caisson shield for the protection of an offshore production platform and, more particularly, a caisson shield for use in an arctic environment for the protection of the offshore structure in iceberg-infested waters which is capable of absorbing the destructive forces of an impact produced by a large iceberg.
More recently, the increased worldwide demand for hydrocarbons, such as oil and natural gas, has necessitated the investigation and exploitation of many new regions throughout the world, both on land and offshore. One of the regions which appears to be extremely promising in its potential for finding hydrocarbon fuels is the offshore arctic and subarctic area in proximity to Canada and Greenland. However, this area calves numerous icebergs each year, the size and shape of which are dependent upon their glacial source, climatic and hydrographic conditions determinative of their survival or deterioration, and the routes and distances over which the icebergs travel and are tracked, frequently over a period of several years, before their eventual destruction in the Northwest Atlantic Ocean. Some of these icebergs reach a magnitude having a mass larger than 180,000 tons and, when impacting against a structure, can produce immense destructive forces.
The offshore production platform, especially the support section thereof which is submerged below the marine surface, if exposed to the impact of large icebergs, in addition to experiencing localized or massive failures in the structure thereof, can to some extent slide along the marine floor which may conceivably cause damage to expensive and difficultly replaceable equipment and pipelines connected to the platform support structure. In order to counteract any substantial or massive foundation or support section failure of the offshore platform, the submerged support section of a self-protecting type of platform should be impressive in its mass and dimensions so that possible failure of structural and foundation components are limited to localized areas which may occasion only a brief and temporary suspension of platform operations when such an impact encountered reaches a maximum or extreme limit. In such an event, there is a definite possibility of damage being imparted to the pipeline and flowline connections to the structure of the offshore platform, and of substantial damage resulting to the structure itself, with the attendant risk of well conductor casings being severed from the offshore platform, in the case of drilling and well completion thereby resulting in a much greater economic loss than would be encountered as the result of a temporary shutdown of the platform.
2. Discussion of the Prior Art
Various solutions to the problems encountered in protecting offshore structures from damage caused by iceberg impact in iceberg-infested waters have been suggested in the prior art. Thus, Pearce et al. U.S. Pat. No. 4,245,929 discloses an offshore structure able to withstand ice forces generated by impinging ice sheets or icebergs in which at least the lower portion of the support structure of the offshore platform includes upper and lower differently sloped conical portions with respect to the walls thereof forming an inclination relative to the horizontal so as to receive and deflect ice masses moving into contact with the platform support structure. The particular structural selection of the conical wall structure is designed to cause the ice to tilt upwardly upon impinging against the support structure and to fragment itself while sliding off the support structure. The type of conical wall structure proposed in Pearce et al. does not appear to be adequate to withstand the impact of extremely large icebergs encountered in arctic or subarctic waters and is primarily intended for the purpose of deflecting relatively thin ice sheets rather than large and massive icebergs.
Howard U.S. Pat. No. 3,766,737 discloses an offshore platform which is encompassed, at a radial distance from the platform, by a circumferentially movable ice trenching machine which will circulate about the platform so as to fragment and remove ice in a circular path at a rate approximately equal to the rate of movement of the ice sheet towards the protected structure. Also this type of protective arrangement for offshore platforms is only adapted to protect the platform from the pressures of ice sheets and does not appear to provide any significant protection against the large destructive forces generated through impact by a massive iceberg.
Challine et al. U.S. Pat. No. 4,142,819 discloses an offshore platform in which the platform is of the gravitytype including a base resting on the marine floor and with an annular shell affording rigidity in the upward direction, such as a circular wall and diaphragms extending about the base portion of the platform so as to provide reinforcement therefore. This type of reinforcing support structure for the base of the offshore platform does not appear to be designed to withstand the impact of icebergs, particularly any relatively large and massive icebergs normally encountered in arctic and subarctic waters and thus would provide inadequate protection for the platform, thereby rendering it unsuitable for use in iceberg-infested waters.